The soil microbe Bacillus thuringiensis (B.t.) is a Gram-positive, spore-forming bacterium characterized by parasporal crystalline protein inclusions. These inclusions often appear microscopically as distinctively shaped crystals. The proteins can be highly toxic to pests and specific in their toxic activity. Certain B.t. toxin genes have been isolated and sequenced, and recombinant DNA-based B.t. products have been produced and approved for use. In addition, with the use of genetic engineering techniques, new approaches for delivering these B.t. endotoxins to agricultural environments are under development, including the use of plants genetically engineered with endotoxin genes for insect resistance and the use of stabilized intact microbial cells as B.t. endotoxin delivery vehicles (Gaertner, F. H., L. Kim [1988] TIBTECH 6:S4-S7). Thus, isolated B.t. endotoxin genes are becoming commercially valuable.
Until the last ten years, commercial use of B.t. pesticides has been largely restricted to a narrow range of lepidopteran (caterpillar) pests. Preparations of the spores and crystals of B. thuringiensis subsp. kurstaki have been used for many years as commercial insecticides for lepidopteran pests. For example, B. thuringiensis var. kurstaki HD-1 produces a crystalline .delta.-endotoxin which is toxic to the larvae of a number of lepidopteran insects.
In recent years, however, investigators have discovered B.t. pesticides with specificities for a much broader range of pests. For example, other species of B.t., namely israelensis and morrisoni (a.k.a. tenebrionis, a.k.a. B.t. M-7, a.k.a. B.t. san diego), have been used commercially to control insects of the orders Diptera and Coleoptera, respectively (Gaertner, F. H. [1989] "Cellular Delivery Systems for Insecticidal Proteins: Living and Non-Living Microorganisms," in Controlled Delivery of Crop Protection Agents, R. M. Wilkins, ed., Taylor and Francis, New York and London, 1990, pp. 245-255). See also Couch, T. L. (1980) "Mosquito Pathogenicity of Bacillus thuringiensis var. israelensis," Developments in Industrial Microbiology 22:61-76; Beegle, C. C., (1978) "Use of Entomogenous Bacteria in Agroecosystems," Developments in Industrial Microbiology 20:97-104. Krieg, A., A. M. Huger, G. A. Langenbruch, W. Schnetter (1983) Z. ang. Ent. 96:500-508, describe Bacillus thuringiensis var. tenebrionis, which is reportedly active against two beetles in the order Coleoptera. These are the Colorado potato beetle, Leptinotarsa decemlineata, and Agelastica alni.
Recently, new subspecies of B.t. have been identified, and genes responsible for active .delta.-endotoxin proteins have been isolated (Hofte, H., H. R. Whiteley [1989] Microbiological Reviews 52(2):242-255). Hofte and Whiteley classified B.t. crystal protein genes into four major classes. The classes were CryI (Lepidoptera-specific), CryII (Lepidoptera- and Diptera-specific), CryIII (Coleoptera-specific), and CryIV (Diptera-specific). The discovery of strains specifically toxic to other pests has been reported. (Feitelson, J. S., J. Payne, L. Kim [1992] Bio/Technology 10:271-275). CryV has been proposed to designate a class of toxin genes that are nematode-specific.
The cloning and expression of a B.t. crystal protein gene in Escherichia coli has been described in the published literature (Schnepf, H. E., H. R. Whiteley [1981] Proc. Natl. Acad. Sci. USA 78:2893-2897). U.S. Pat. No. 4,448,885 and U.S. Pat. No. 4,467,036 both disclose the expression of B.t. crystal protein in E. coli. U.S. Pat. Nos. 4,990,332; 5,039,523; 5,126,133; 5,164,180; and 5,169,629 are among those which disclose B.t. toxins having activity against lepidopterans. U.S. Pat. Nos. 4,797,276 and 4,853,331 disclose B. thuringiensis strain tenebrionis which can be used to control coleopteran pests in various environments. U.S. Pat. No. 4,918,006 discloses B.t. toxins having activity against dipterans. U.S. Pat. No. 5,151,363 and U.S. Pat. No. 4,948,734 disclose certain isolates of B.t. which have activity against nematodes. Other U.S. Patents which disclose activity against nematodes include 5,093,120; 5,236,843; 5,262,399; 5,270,448; 5,281,530; 5,322,932; 5,350,577; 5,426,049; and 5,439,881. As a result of extensive research and investment of resources, other patents have issued for new B.t. isolates and new uses of B.t. isolates. See Feitelson, J. S., J. Payne, L. Kim (1992) Bio/Technology 10:271-275 for a review. However, the discovery of new B.t. isolates and new uses of known B.t. isolates remains an empirical, unpredictable art.
Regular use of chemical control of unwanted organisms can select for chemical resistant strains. Chemical resistance occurs in many species of economically important insects and has also occurred in nematodes of sheep, goats, and horses. The development of chemical resistance necessitates a continuing search for new control agents having different modes of action. The subject invention pertains specifically to materials and methods for the identification of B.t. toxins active against nematodes or coleopteran pests. Of particular interest among the coleopteran pests is the corn rootworm.
In recent times, the accepted methodology for control of nematodes has centered around the drug benzimidazole and its congeners. The use of these drugs on a wide scale has led to many instances of resistance among nematode populations (Prichard, R. K. et al. [1980] "The problem of anthelmintic resistance in nematodes," Austr. Vet. J. 56:239-251; Coles, G. C. [1986] "Anthelmintic resistance in sheep," In Veterinary Clinics of North America: Food Animal Practice, Vol 2:423-432 [Herd, R. P., Eds.] W. B. Saunders, New York). There are more than 100,000 described species of nematodes.
A small number of research articles have been published about the effects of delta endotoxins from B. thuringiensis species on the viability of nematode eggs. Bottjer, Bone and Gill ([1985] Experimental Parasitology 60:239-244) have reported that B.t. kurstaki and B.t. israelensis were toxic in vitro to eggs of the nematode Trichostrongylus colubriformis. In addition, 28 other B.t. strains were tested with widely variable toxicities. Ignoffo and Dropkin (Ignoffo, C. M., Dropkin, V. H. [1977] J. Kans. Entomol. Soc. 50:394-398) have reported that the thermostable toxin from Bacillus thuringiensis (beta exotoxin) was active against a free-living nematode, Panagrellus redivivus (Goodey); a plant-parasitic nematode, Meloidogyne incognita (Chitwood); and a fungus-feeding nematode, Aphelenchus avena (Bastien). Beta exotoxin is a generalized cytotoxic agent with little or no specificity. Also, H. Ciordia and W. E. Bizzell ([1961] Jour. of Parasitology 47:41 [abstract]) gave a preliminary report on the effects of B. thuringiensis on some cattle nematodes.
There are a number of beetles that cause economic damage. For example, Chrysomelid beetles such as flea beetles and corn rootworms and curculionids such as alfalfa weevils are particularly important pests. Flea beetles include a large number of small leaf feeding beetles that feed on the leaves of a number of grasses, cereals and herbs. Flea beetles include a large number of genera (e.g., Altica, Apphthona, Argopistes, Disonycha, Epitrix, Longitarsus, Prodagricomela, Systena, and Phyllotreta ). The flea beetle, Phyllotreta cruciferae, also known as the rape flea beetle, is a particularly important pest. Corn rootworms include species found in the genus Diabrotica (e.g., D. undecimpunctata undecimpunctata, D. undecimpunctata howardii, D. longicornis, D. virgifera and D. balteata). Corn rootworms cause extensive damage to corn and curcubits. The western spotted cucumber beetle, D. undecimpunctata undecimpunctata, is a pest of curcubits in the western U.S. Alfalfa weevils (also known as clover weevils) belong to the genus, Hypera (H. postica, H. brunneipennis, H. nigrirostris, H. punctata and H. meles), and are considered an important pest of legumes. The Egyptian alfalfa weevil, H. brunneipennis, is an important pest of alfalfa in the western U.S.
Approximately $250 million worth of insecticides are applied annually to control corn rootworms alone in the United States. In the Midwest, $60 million and $40 million worth of insecticide were applied in Iowa and Nebraska, respectively, in 1990. It has been estimated that the annual cost of insecticides to control corn rootworm and the annual crop losses caused by corn rootworm damage exceeds a total of $1 billion in the United States each year (Meycalf, R. L. [1986] in Methods for the Study of Pest Diabrotica, Drysan, J. L. and T. A. Miller [Eds.], Springer-Verlag, New York, N.Y., pp. vii-xv). The corn rootworm (Diabrotica spp.) is a coleopteran pest which causes extensive damage to corn crops each year due to root feeding by the larvae. Three main species of corn rootworm, Western corn rootworm (Diabrotica virgifera virgifera), Northern corn rootworm (Diabrotica barberi), and Southern corn rootworm (Diabrotica undecimpunctata howardi) cause varying degrees of damage to corn in the United States. Even with insecticide use, rootworms cause about $750 million worth of crop damage each year, making them the most serious corn insect pest in the Midwest.
The life cycle of each Diabrotica species is similar. The eggs of the corn rootworm are deposited in the soft. Newly hatched larvae (the first instar) remain in the ground and feed on the smaller branching corn roots. Later instars of Western and Northern corn rootworms invade the inner root tissues that transport water and mineral elements to the plants. In most instances, larvae migrate to feed on the newest root growth. Tunneling into roots by the larvae results in damage which can be observed as brown, elongated scars on the root surface, tunneling within the roots, or varying degrees of pruning. Plants with pruned roots usually dislodge after storms that are accompanied by heavy rains and high winds. The larvae of Southern corn rootworm feed on the roots in a similar manner as the Western and Northern corn rootworm larvae. Southern corn rootworm larvae may also feed on the growing point of the stalk while it is still near the soil line, which may cause the plant to wilt and die.
After feeding for about 3 weeks, the corn rootworm larvae leave the roots and pupate in the soft. The adult beetles emerge from the soft and may feed on corn pollen and many other types of pollen, as well as on corn silks. Feeding on green silks can reduce pollination level, resulting in poor grain set and poor yield. The Western corn rootworm adult also feeds upon corn leaves, which can slow plant growth and, on rare occasions, kill plants of some corn varieties.
Current methods for controlling corn rootworm damage in corn are limited to the use of crop rotation and insecticide application. However, economic demands on the utilization of farmland restrict the use of crop rotation. In addition, an emerging two-year diapause (or overwintering) trait of Northern corn rootworms is disrupting crop rotations in some areas.
The use of insecticides to control corn rootworm and other coleopteran pests also has several drawbacks. Continual use of insecticides has allowed resistant insects to evolve. Extremely high populations of larvae, heavy rains, and improper calibration of insecticide application equipment can result in poor control. Insecticide use often raises environmental concerns such as contamination of soil and of both surface and underground water supplies. Working with insecticides may also pose hazards to the persons applying them.
At the present time there is a need to have more effective means to control the many nematodes and the corn rootworm that cause considerable damage to susceptible hosts and crops. Advantageously, such effective means would employ specific biological agents.
Bacillus thuringiensis toxins which are active against nematodes and corn rootworm are now known. However, to date, the method for isolating the responsible toxin genes has been a slow empirical process. That is, for a given active B.t. isolate, there is currently no rapid systematic method for identifying the responsible toxin genes or for predicting the activity of a given B.t. isolate. The subject invention helps to eliminate the empirical nature of finding certain B.t. insecticidal protein toxin genes. Although the process is still highly unpredictable, this invention facilitates expedient identification of potentially new commercially valuable insecticidal isolates and endotoxin genes.
A recent report of similar methods has appeared (see Carozzi, N. B., V. C. Kramer, G. W. Warren, S. Evola, G. Koziel [1991] Appl. Env. Microbiol. 57(11):3057-3061). This report does not disclose or suggest the specific primers and probes of the subject invention for nematode-active and corn rootworm-active toxin genes. U.S. Pat. No. 5,204,237 describes specific and universal probes for the isolation of B.t. toxin genes. This patent, however, does not describe the probes and primers of the subject invention.